1. Field of the Invention
The present invention relates generally to magnetic field sensors, and more specifically, to a bias current circuit that is stable across temperature and process variations.
2. Background of the Invention
Hall effect sensors and other semiconductor magnetic field sensors are widely used in applications in which it is desirable to provide a measurement of DC magnetic fields and relatively low frequency AC magnetic fields that are not otherwise easily sensed with coils or other antennas. Such applications include position and motion sensors for both linear and rotational motion, power supply and motor control applications in which the transformer or motor fields are detected, audio speaker applications in which the strength of the speaker's signal-induced field is detected, and lighting controllers for high-frequency energized lamps, such as sodium lamps.
Hall effect sensors operate by providing a layer of semiconductor material with a bias current applied across one axis and sensing a voltage across the other axis. When a magnetic field is present, the uniformity of the current in the layer of material is distorted, causing non-uniform voltage distribution along the material and a differential voltage to appear across a pair of sensing terminals.
Since any variation in the bias current, including noise, is reflected in the output of the Hall effect sensor, the bias current source must be low-noise and stable over environmental changes such as temperature. Further, since temperature changes that affect the bias current source produce corresponding changes in the Hall effect sensor output, a temperature stable bias current source must be provided. While circuits employing Hall effect sensors may be calibrated, for integrated circuit applications, it is desirable to produce a solution that has low complexity and requires few or no external components.
Therefore, it would be desirable to provide a low noise bias current source for a semiconductor magnetic field sensor circuit having reduced noise and little variation over process and temperature.